RECENT DEVELOPMENTS IN ULTRAFAST TIME-RESOLVED SPECTROSCOPY OF ELECTRONICALLY EXCITED STATES

Hiro-o Hamaguchi

Deaprtment of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo,
Tokyo 113-0033, Japan (e-mail:hhama@chem.s.u-tokyo.ac.jp; FAX:81-3-3818-4621)

Structure and dynamics of electronically excited states has always been a matter of great concern in Chemical Physics. Luminescence spectroscopy, the central theme of this Conference, is obviously one of the most powerful means for studying excited states. However, there are many kinds of interesting excited states that do not emit luminescence. Furthermore, the structural information obtained from luminescence spectroscopy is rather limited. These difficulties can be well compensated for by time-resolved vibrational (infrared and Raman) spectroscopy which looks directly into the structure of excited states. In the present lecture, I review our recent efforts for developing new time-resolved vibrational spectroscopies and their applications to some electronically excited states of key importance.

Picosecond Time-resolved Raman spectroscopy. A transform-limited picosecond time-resolved Raman spectrometer was constructed based on an amplified dye laser system [1]. Time resolution of 2.2 ps and wavenumber resolution of 3.2 cm-1 have been achieved. The system was used to study the vibrational cooling dynamics of S1 trans-stilbene (tSB) in solution and the detailes of the solute/solvent interactions has been elucidated [2]. The analysis of the Raman band shape of S1 tSB has led to a new insight into the mechanism of the trans-perpendicular isomerization [3].
Picosecond time-frequency two-dimensional multiplex CARS spectroscopy. Combining a broadband multiplex CARS system with a streak camera, we constructed a time-frequency two-dimensional multiplex CARS spectrometer [4]. It measures a set of picosecond time-resolved CARS spectra in hundreds of seconds. Picosecond time-resolved CARS spectra of diphenyl acetylene (DPA) in the S2 and S1 state have been obtained [5]. The observed CARS frequency indicates that the central C-C bonding of S1 DPA is double-bond like rather than triple: S1 DPA is not an acetylene.
Nanosecond time-resolved dispersive infrared spectroscopy. An ultrasensitive time-resolved infrared spectrometer has been constructed that can detect an infrared intensity change of as small as one part in a million [6]. Time-resolution is limited by the detector response time which is 50 ns at present. The infrared spectra of the S1 and T1 states of N,N-dimethylamino-benzonitrile (DMABN) have been obtained [7]. The CT strucure of the excited DMABN has been elucidated.

1. K. Iwata, S. Yamaguchi and H. Hamaguchi (1993) Rev. Sci. Instru. 64, 2140-2146.
2. K. Iwata and H. Hamaguchi (1997) J. Phys. Chem. 106, 11-17.
3. H. Hamaguchi and K. Iwata (1993) Chem. Phys. Lett. 208, 465-470.
4. T. Tahara abd H. Hamaguchi (1994) Rev. Sci. Instru. 65, 3332-3338.
5. T. Ishibashi and H. Hamaguchi (1998) J. Phys. Chem. 102, 2263-2269.
6. T. Yuzawa, C. Kato, M. W. George and H. Hamaguchi (1994) Appl. Spectrosc. 48, 684-690.
7. M. Hashimoto and H. Hamaguchi (1995) J. Phys. Chem. 99, 7875-7877.

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